1. Field of the Invention
The present invention relates to a resonator used for NMR detection and, more particularly, to a resonance signal detector with enhanced sensitivity.
2. Description of Related Art
A conventional NMR detector is illustrated in FIG. 15. This detector has a bobbin 2 accommodating a sample tube 1. A saddle coil 3 is formed from wire around the bobbin 2. The ends of two extraction lines from the saddle coil 3 are connected together via a capacitor 4. In this way, an LC resonance circuit is formed by the saddle coil 3 and capacitor 4. This resonance circuit is connected with an external tuning and matching circuit (not shown). This resonance circuit produces an RF magnetic field that is directed at a sample within the sample tube 1. The resulting resonance signal is detected by the saddle coil 3.
In this conventional detector coil, an electrical current flows through the wound lead wire. Therefore, electrical resistance results in energy loss. At room temperature, the Q factor is as low as around 200. Furthermore, the detector coil produces a strong RF electric field within the sample space. Therefore, the dielectric loss due to the sample is large. This further deteriorates the Q factor. In addition, the RF electric field produced in the sample space dielectrically heats the sample.
The present invention is intended to provide a technique for producing a high Q factor even at room temperature, producing enhanced sensitivity by weakening the intensity of the RF electric field leaking into the sample space, and reducing heating of the sample by decreasing dielectric heating due to the RF electric field. For this purpose, a detector coil, in accordance with the present invention, is covered with a dielectric material, especially a ferroelectric material, or buried in a ferroelectric material. An electric circuit is attached to the outside. In this way, a high-sensitivity detector coil is obtained. A distributed constant coil that is brought into resonance by standing electromagnetic waves can have a high Q factor but the resonance frequency is too high to be used in NMR. Accordingly, the coil is covered with a ferroelectric material. This increases the dielectric constant of the surroundings of the coil, which in turn decreases the propagation speed of electromagnetic waves. In consequence, the wavelength of electromagnetic waves shortens. As a result, a resonance frequency that can be used in NMR and derived from a distributed constant coil is obtained, the coil being brought into resonance by standing waves.
In the coil of this structure, resonance modes are degenerate and so it is difficult in practice to produce RF magnetic field distribution necessary for measurement. However, the necessary RF magnetic field distribution can be generated by attaching an electric circuit to the coil so as to circumvent degeneracy. Shortening the wavelength of an electromagnetic field is equivalent to reducing the length while the width of the conductors passing electric current remains the same. Therefore, the electrical resistive component decreases, increasing the Q factor. This enhances the sensitivity. Furthermore, the ferroelectric material reduces the impedance of the coil. Where the same intensity of RF magnetic field is produced, the RF voltage generated by the current necessary to produce it decreases. Consequently, the intensity of the RF electric field produced in the sample space and applied to the sample decreases. This alleviates the dielectric loss due to the sample. Hence, heating of the sample and decrease of the Q factor can be prevented. A high sensitivity not dependent on the dielectric loss factor of the sample is obtained.